1. Field of the Invention
This invention concerns a process for the production of ceramic plutonium-uranium nuclear fuel in the form of sintered pellets made from a fine-grained starting material containing plutonium and uranium as oxide, carbide or nitride.
2. Description of the Prior Art
In the production of ceramic nuclear fuel, i.e. ceramic material from uranium, plutonium or thorium, or mixtures of these metals, or solid solutions of such mixtures of metals, wherein all components are present in the form of oxides, nitrides or carbides, conventionally the usually cylindrical fuel pellets are obtained by sintering pressed "green powders", as this is described, e.g., in "Nuclear Energy Maturity. Proceedings of the European Nuclear Conference Vol. 7 `Fuel Fabrication`. Progress in Nuclear Energy, Pergamon Press (1976)".
In the case of plutonium-containing fuel, as, e.g., uranium with about 3% plutonium for the production of oxide for use in a Light Water Reactor (LWR) Pressurised Water Reactor, PWR), Boiling Water Reactor (BWR) or Advanced Gas-cooled Reactor (AGR), or uranium with 10-14% plutonium in the form of mixtures of oxide, carbide or nitride for use in a Fast Breeder Reactor (FBR) cooled with liquid sodium or a gas, it is usual to blend the two pulverulent components which separately contain the uranium and plutonium in the form of oxide, or to mix the uranium oxide and the plutonium oxide together with finely divided carbon in order to obtain the carbide and the nitride.
From these starting powder mixtures cylindrical green pellets in hydraulic or mechanical presses are pressed. The green pellets are then baked in order to result in a dense sintered pellet. In the case where in the manufacture of the green pellets the oxide was mixed with carbon, a preliminary heating step may be interposed before the baking during which the carbon reacts with the oxides of plutonium and uranium and forms the carbide. The thus obtained carbide material is then granulated by comminution and ground and finally, as before, pressed and sintered in order to obtain the required final density.
The final sintering used with green pellets mixed from oxides or with the re-shaped carbide pellets is important for the manufacture of dense pellets with a homogeneous distribution of plutonium in the uranium structure. This homogenisation takes place by self-diffusion and is necessary so that no localised regions of above average plutonium concentration should occur.
The good sintering properties required for achieving a high final density and a complete diffusion of the plutonium in the uranium matrix are dependent on the characteristics of the powder forming the initial mixture, such as particle size, particle surface, particle shape, flowability and the quality of the mixture itself. In order to obtain sintered pellets of high quality, the uranium or plutonium oxide powder with or without carbon, or the separately won powders of uranium and plutonium carbides must be ground optimally and mixed extremely carefully. In certain cases, the final grinding and the mixing of the powders are undertaken in a single process step while in other cases the powders are first ground to the required quality and then mixed. The grinding and mixing of plutonium-containing powders give rise to difficulties.
Because of its toxicity, its radioactivity and the danger of ingestion, plutonium material can only be handled and prepared under the strictest controls in a sealed room, such as a radiologically protected room or in an .alpha.-tight cell. Some industrial nations have issued fabrication guidelines for the production of plutonium-containing fuels which comply with the above-mentioned principles.
In the manufacture of ceramic fuel pellets, it is particularly the plutonium-containing dust arising in the preparation of the powder that leads to special difficulties, because without special precautionary measures the operating personnel receive, or could receive, impermissibly high radiation doses. Such radiation fields are also caused in part by a byproduct of plutonium, namely americium, which emits hard gamma rays and can cause a direct irradiation of the service personnel or the maintenance personnel through the dust which coats the apparatus and the interior surfaces of the radiation-protected rooms into which latter personnel must from time to time enter in protective clothing for the maintenance of apparatus. Further difficulties are caused by lost plutonium, i.e., plutonium adhering to surfaces and retained in the filters.
It is anticipated that these problems will increase, particularly because of the increase in the radioactivity of plutonium caused by the longer residence time (higher burn-up) in "producing" reactors, to which one should then still add the spontaneous neutron emission (spontaneous fission) of Pu-238 (.alpha., n) and the gamma emission of plutonium 236 (.gamma.) which increases with increasing recovery of plutonium materials.
Spherical fuel particles may be produced in wet chemical processes. Of these, the most important are:
The Sol-gel process which has been described, e.g., in:
(L1) Proc. Panel for ceramic nuclear fuels, 6-10 May 1968 Vienna, IAEA PA1 "Sol-gel processes for ceramic nuclear fuels"; PA1 (L2) U.K. Patent Spec. No. 1,175,834 (24.12.1967) "Improvements in or relating to the chemical production of metal containing materials as particles or other configurations" and PA1 (L3) Julich Report, Jul. 1229, 1975; PA1 (L4) German Patent Spec. No. 2,059,093 "Process for the production of spherical particles which contain a metallic oxyhydrate, metallic oxyhydrate with carbon . . . , from an aqueous solution" and in PA1 (L5) Forthmann R. et al, in Jul-655-RW (April 1970) "Investigation on the preparation of UO.sub.2 microspheres by internal gelation of a UO.sub.2 sol and from a uranium (VI) solution", and PA1 (L6) Stratton R, and Bischoff K in Op. cit ref. 1, vol. 3 "The mixed carbide fuel programme at EIR". PA1 (i) a mixture of (a) an oxide, finely divided carbon-containing oxide, carbide, nitride, oxycarbide, or carbonitride of plutonium, in the form of microspheres obtained by a wet chemical process, and (b) a like compound of uranium in a form selected from such microspheres and fine-grained to pulverulent materials; or PA1 (ii) a like compound of a plutonium-uranium solid solution of which at least 1% is plutonium, in the form of microspheres obtained by a wet chemical process, alone or in admixture with a material selected from the said plutonium-compound microspheres, the said uranium-compound microspheres and the said fine-grained to pulverulent materials,
The precipitation process, known from:
The internal gelation process, described in:
The EIR carbide process which is described in
These wet chemical or gelation processes may all be used for the purpose of directly obtaining ceramic sintered material from uranium and plutonium, i.e. without grinding, mixing and powder pressing, in the form of microspheres of oxide, carbide, carbide or nitride for use as fuel in reactors. Such a special process is described e.g. in Swiss Patent Specification 581,890 (REACTOR CENTRUM NEDERLAND) "Process for converting a spherical sol-gel fissile material by sintering to a spherical nuclear fuel material containing oxide, carbide or carbonitride".
Accordingly, ceramic uranium-plutonium nuclear fuel is available in the form of microspheres producible by a wet chemical method or in the form of sintered pellets which latter are significantly more advantageous in use than microspheres but the manufacture of which is difficult and has problems that can only be solved by corresponding expenditure.